The use of thick-film resistors, capacitors, etc. parts in microcircuits is becoming of increasing importance in the electrical and electronic field. These thick-film components which comprise a layer of ink or paste which may be conductive, partially conductive, semiconductive or nonconductive in nature are deposited on a ceramic substrate by a process which is similar in nature to the silk-screening method whereby a pattern of films is laid down to form conductors, dielectrics, resistors, capacitors or semiconductors. Following the deposition of the film on the substrate, the resulting material is then fired to a temperature usually ranging from about 500.degree. to about 1000.degree. C. or more in air whereby the film is firmly affixed to the substrate. The resultant paste or ink substrate combination can form a microcircuit of passive components and, in addition, if so desired, discrete active components such as transistors or integrated circuit chips can be attached separately to form a thick-film hybrid device.
As hereinbefore set forth, the use of thick-film items or products is becoming more important due to the advantages which these items offer over other technology such as discrete parts, printed circuits, thin films, etc. For example, the designs which are used which have thick-film networks are easy, quick and flexible with low development costs and offer the design freedom and variety of parameter values which are normally available with discrete paerts. Furthermore, circuits formed from thick films can combine many types of components such as high value capacitors, resistors, etc. which are not possible with monolithic products. In addition, the method of preparation of thick-film devices is simple inasmuch as the screen printing and heating processes are easy to control and automate. This is in contradistinction to thin-film networks which require a great degree of care in the sputtering and evaporating processes. The operation advantages which are possible when utilizing thick-film devices include high reliability which results from the use of fewer interconnection points. Furthermore, in contrast with discrete parts, the thick-film devices have improved resistance-matching and temperature-tracking capabilities.
All of the above-enumerated advantages will permit the use of thick-film devices in consumer radio and television products as well as in computers and in industrial electronic devices. These thick-film devices such as resistor networks may be used to replace the carbon resistor while hybrid modules including a thick-film device may be used in television circuits for the horizontal and vertical oscillators, high-voltage dividers and chroma signal processors. Additional uses for these devices are found in telephones, two-way radios, multiplexers, insulators, voltage regulators and heating aids. Likewise, these devices may also be used in industrial control systems such as analog-to-digital and digital-to-analog converters, operation amplifiers, servo amplifiers, power amplifiers and power supply regulators, while in the automotive field hybrid thick-film devices may be used in fuel injection systems. It is thus readily apparent that thick-film devices find a wide variety of uses in many fields.
The silk-screen conductor pastes which are currently in use are produced by combining a noble metal pigment such as gold, silver, platinum, palladium, etc. with a powder glass mixture, an organic vehicle and an organic binder. Thereafter, the paste is silk-screened onto a ceramic substrate and thereafter taken through a firing cycle at a temperature in the range hereinbefore set forth which first burns off the organic vehicle and thereafter melts the glass frit. On cooling, the product is a distribution of metal pigment in a glassy matrix which possesses an electrical conductivity sufficient to produce minimal and predictable resistance in the electric circuit.
The most common commercial inks which are used in thick-film circuitry, as hereinbefore set forth, are based on the noble metals. However, due to the volatility of the prices of these noble metals as well as the availability thereof, there is a strong incentive to replace the noble metal pigments with a nonnoble metal pigment which would result in less costly conductors. While several nonnoble metal systems are currently in use, there are certain disadvantages inherent in these systems which prevent a wide acceptance of the system. One drawback in using these nonnoble conductive metals such as nickel or copper has been that these metals are subject to a relatively ready oxidation of the metal, thereby reducing the conductivity of the metal to a point where it is insufficient in conductive properties to be useful in microcircuits. Another disadvantage is that the inks or conductive surfaces possess a poor solderability. Currently, copper-based systems do possess good electrical conductivity. However, the firing of the ink must be accomplished either in an inert atmosphere or at a low temperature such as a maximum of 700.degree. C. To overcome this relatively low temperature firing, it has been necessary to incorporate a selective oxidatable material into the ink. U.S. Pat. No. 4,122,232 discloses a paste which is used in forming a base metal thick-film electrical conductor which comprises mixing the paste consisting of a base metal powder such as nickel, copper, cobalt or mixtures thereof with a boron powder and a vehicle and a glass frit, said vehicle comprising an organic compound of the type well known in the art. A somewhat similar conductive ink is disclosed in U.S. Pat. No. 4,322,316 which discloses a thick-film conductor paste consisting of boron, copper oxide and a glass frit as well as an inert vehicle.
Various U.S. patents have shown different inks. For example, U.S. Pat. No. 3,663,276 deals with inks which are used as resistors having a resistance greater than 100,000 ohms per square. However, this reference uses noble metals or noble metal oxides with nonnoble metals of given concentrations. The nonnoble metals oxidize upon firing, thus becoming nonconductive in nature and providing the desired high resistivity. Other U.S. patents such as U.S. Pat. Nos. 3,843,379, 3,811,906 and 3,374,110 describe utilizing a noble metal that is mixed with a vitreous frit, an organic binder, a solvent and is thereafter fired in an air atmosphere at an elevated temperature. These patents describe the use of noble metals such as gold, silver, palladium or mixtures thereof. While certain U.S. patents such as U.S. Pat. Nos. 3,647,532 and 2,993,815 describe the use of nonnoble metals as conductive inks, it is necessary that these inks utilize a furnace with an accurately controlled special type atmosphere. For example, in the former patent, the firing is effected in an essentially neutral or inert atmosphere, except that it contains sufficient oxygen and claims that the upper limit of the oxygen which is present is 0.1% by volume. Further, this reference also utilizes a reducing agent within the ink such as hydrazine hydrate which when decomposed at elevated temperatures releases hydrogen and reacts with excess oxygen, thus preventing oxidation of the base matter in the essentially neutral atmosphere. The purpose of the low oxygen content in this patent is to burn off the binder, but it cannot be any higher inasmuch as it will oxidize the conductive metal and render the ink electrically nonconductive. By utilizing this inert or essentially neutral atmosphere, the atmosphere is identical to a rare gas such as neon, argon, krypton, xenon, radon, etc. which show practically no tendency to combine with other elements. Therefore, an inert atmosphere is neither oxidizing nor reducing which is in contradistinction, as hereinafter set forth in greater detail, to the oxidizing atmosphere of the present invention. U.S. Pat. No. 2,993,815 hereinabove cited uses two firing operations. The first firing is effected in an air, oxygen or mixed oxygen and inert gas environment so as to form the glass-metal bond. Following this, the second firing is effected in a reducing atmosphere possessing a critical composition of nitrogen, hydrogen and small amounts of oxygen to reduce the oxidized metal. Nonnoble metals such as copper, nickel, alloys of nickel and copper or iron when fired in an air atmosphere at 840.degree. C. are known to oxidize rapidly and therefore will no longer be able to be utilized as conductive metals.
It is also known that reducing agents can be added to the glass frit. However, this produces spotty conduction zones. The addition of antimony, chromium, charcoal or other oxygen scavengers can be mixed or blended into the conductive ink, but on firing reduction is nonuniform and will tend to occur only where the oxygen scavenger is present. U.S. Pat. No. 3,711,428 describes the mixing of charcoal with the ink. However, this action is taken to prevent blistering or cratering of the resistor, the charcoal burning off and thus leaving the metal exposed for oxidation. While this does not cause problems for the noble metal, there is substantial oxidation of nonnoble metals such as copper. Another U.S. patent, namely U.S. Pat. No. 2,795,680, utilizes a ceramic base to which is bonded a cross-linked epoxy resin and a conductive and nonconductive powder. The resin is cross-linked at 250.degree. C. which is well below the firing temperature which is utilized in the present invention. In the event that resistors need to be cofired, the conductor ink could not withstand the higher temperature.
In addition to the aforementioned references, U.S. Pat. No. 4,079,156 describes a conductive metal pigment which is prepared by alloying a nonnoble conductive metal with an oxidizable material followed by mixing the resulting alloy with a vitreous frit and an organic vehicle to form an ink. The ink is then screened onto a substrate followed by firing said ink in an oxidizing atmosphere containing at least 20% by volume of oxidation in a temperature in excess of about 500.degree. C. The firing is effected for a period of time which is sufficient to utilize the oxidizable material without oxidation of the nonnoble metal. The oxidizable material is present in the alloy in an amount within the range of from about 0.1 to about 10% by weight of the alloy. The resulting ink or conductive pigment will hereinafter be used in the preparation of a thick-film device, the conductivity of the conductive metal portion of the pigment being retained in an amount sufficient to permit the conductive pigment to be used in microcircuitry.
U.S. Pat. No. 3,943,168 discloses conductor compositions comprising nickel borides in which the compositions are finely divided inorganic powders comprising one or more compounds of nickel such as a mixture of nickel boride and nickel boride-silicide. It is also stated in this patent that the compositions may contain nickel metal powder in which the nickel powder may comprise up to 8% of the total weight of the nickel and nickel compounds present. Likewise, U.S. Pat. No. 4,130,854 discloses a borate-treated nickel pigment for metalizing ceramic, the borate coating forming a glass on the surface of the nickel powder, the borate forming an oxidation-resistant film which aids in the adhesion of the nickel to the substrate.
Another U.S. patent, namely U.S. Pat. No. 3,671,291, discloses a method for preparing an alloy which comprises placing a metal salt and boric acid into a bath and thereafter inserting a substrate into the bath to produce a very thin conductive film. However, this technique could not be utilized for the thick film conductive surface of the present invention inasmuch as specific patterns of conductors could not be applied, the deposition rate utilizing this technique would be much lower and therefore would require a relatively long preparation time. Furthermore, such thin films would be very difficult to solder. U.S. Pat. No. 3,391,010 is very similar in nature to this U.S. patent and the same disadvantages would also apply thereto. U.S. Pat. No. 3,181,981 discloses a method for making a semiconductor device. The alloy sheet described in this patent must be diffusion bonded to the substrate. This step would be very difficult to accomplish in a thick film application inasmuch as the substrate is usually a high temperature ceramic. The need to deform the foil restricts the boron content of the alloy to low values, and will also decrease the oxidation resistance, thus making it unapplicable for thick film ink applications. Bonding, if possible, must be performed in an inert atmosphere. Likewise, the ink would be difficult to solder. U.S. Pat. No. 3,872,360 is drawn to capacitors with nickel-containing electrodes produced from a nickel alloy or a physical mixture of a nickel alloy plus a noble metal. However, the problem of solderability does not enter into this inasmuch as the nonnoble portion of the system is not solderable.
U.S. Pat. No. 3,843,350 physically mixes a platinum/copper alloy with at least 50% silver to form a conductor. The examples show that the solderability of the mixture drops from excellent to good when the copper content of the alloy increases, thus a base metal above about 25% would not be expected to be solderable. Another U.S. patent which discloses an alloy is U.S. Pat. No. 4,081,710. However, this patent is drawn to a very high temperature igniter or sparking device, particularly for gas turbine engines and was never intended to pertain to solder bond inasmuch as solder would melt well below the operating temperature described therein.
Another disadvantage which is inherent in nonnoble metal-based thick-film conductor inks is a difficulty in soldering the inks to surfaces to form the desired circuits. Inasmuch as noble-based inks are relatively expensive and thus add to the overall cost of the finished product, a present need exists for a moderately priced conductor ink which is readily solderable in a relatively simple manner utilizing conventional methods.
As will hereinafter be shown in greater detail, we have now discovered that a conductive and solderable pigment-coated thick film surface which possesses excellent solderability and conductive characteristics may be prepared according to the method herein set forth. It was totally unexpected that an alloy powder for conductor applications having a non-noble metal content up to 75% in conjunction with a noble metal would possess excellent solderable characteristics, especially in view of prior patents which indicated that an alloy containing greater than 25% of nonnoble metal would possess poor solderable characteristics.